Automotive systems continuously or periodically monitor numerous fluids to ensure that performance continues as expected. There are many fluid properties that can be monitored using techniques such as compositional analysis, quantitative analysis and contaminant concentration. Examples of these include monitoring for excess soot in lubricant oil, for the presence of water or methanol in gasoline, for the remaining quantity of lubricant, and the like.
The measurement of capacitance or complex impedance between two parallel plate electrodes or between two coaxial tube electrodes can be used to quantify certain fluid properties. The fluid is typically passed through a gap maintained by the parallel plates and a dielectric constant of the fluid is determined as it is passed between the plates. Monitoring the fluid""s dielectric constant can be used to detect changes in the fluid, indicating the presence of, for example, contaminants or additives. Alternatively, the measured capacitance may be used to determine the level of fluid in a container.
Some previous capacitive sensors of fluid properties have used plastic components to fabricate the sensor. For sensors that include plastic components, the dielectric constant (i.e., complex impedance) is known to be nonlinear as a function of temperature. To compensate for this non-linear behavior, capacitive sensors fabricated with plastic components require additional data collection, which adds to the overall operating and manufacturing costs.
Other types of capacitive sensors use rivets and spacer rings to separate opposing carrier plates. The spacer rings are positioned onto rivet shafts between the plates to form a gap. The rivets and spacer rings must be electrically insulated and are necessarily positioned outside the areas of the metal capacitor coatings or claddings, thereby adding to the overall costs to manufacture the sensor. Moreover, the structural stability of the sensor relies on the number of rivets and the spacing of the rivets from each other. The rivets cannot assure in all instances that the carrier plates will not bend or warp during use. Such warping is undesirable because it varies the spacing between the capacitor plates resulting in variability and error.
To optimize the signal-to-noise ratio for these types of capacitive sensors, the gap (or distance) between the parallel plates needs to be minimized. However, if the gap becomes too small, the fluid flow within the gap is hindered and as a result, the response time of the sensor increases. Moreover, there is a propensity for the gap to trap material and further hinder the fluid flow.
Even if the above noted problems are overcome, it is always a challenge to manufacture a device with a small gap economically and reproducibly.
A sensor comprising a substrate consisting essentially of a non-conductive material; a first electrode, and a second electrode disposed on a first surface of the substrate, wherein the first electrode comprises a first major portion traversing a length of the substrate and a finger extending from the major portion, wherein the second electrode comprises a second major portion traversing the length of the substrate and a finger extending from the second major portion, wherein the first electrode finger extends toward the second electrode major portion and the second electrode finger extends toward the first electrode major portion and is substantially parallel to the first finger; and a third electrode connected to a ground, wherein the third electrode is interposed between and about the first and second electrodes.
In another embodiment, a sensor for measuring a characteristic of a fluid comprises a first, a second and a third ceramic substrate. A first capacitive sensor is sandwiched between the first and second substrates. The first capacitive sensor comprises a first electrode, a second electrode and a third electrode, wherein a portion of the first and second electrodes form complementary parallel finger pairs and wherein the third electrode is grounded and is interposed between and about the first and second electrodes. A second capacitive sensor is sandwiched between the second and third substrates. The second capacitive sensor comprises a fourth electrode, a fifth electrode and a sixth electrode, wherein a portion of the fourth and fifth electrodes form complementary parallel finger pairs, and wherein the sixth electrode is grounded and is interposed between and about the fourth and fifth electrodes. Circuitry means are connected to the first and second capacitive sensors for producing an output signal based on an electrical field generated by the finger pairs.
A system for detecting a change in fluid properties comprises a power supply, a source circuit and an output circuit. The source circuit includes a sensor, wherein the sensor comprises a substrate consisting essentially of a non-conductive material. A first electrode, and a second electrode are disposed on a first surface of the substrate, wherein the first electrode comprises a first major portion traversing a length of the substrate and a finger extending from the major portion. The second electrode comprises a second major portion traversing the length of the substrate and a finger extending from the second major portion, wherein the first electrode finger extends toward the second electrode major portion and the second electrode finger extends toward the first electrode major portion and is substantially parallel to the first finger. A third electrode is connected to a ground, wherein the third electrode is interposed between and about the first and second electrodes. The output circuit comprises amplification means for amplifying a differential signal to produce an output signal that is proportional to a change in an impedance property of a fluid.
A process for measuring the capacitive properties of a fluid comprises attaching a sensor to a fluid container. The sensor comprises a substrate consisting essentially of a non-conductive material. A first electrode and a second electrode are disposed on a first surface of the substrate, wherein the first electrode comprises a first major portion traversing a length of the substrate and a finger extending from the major portion. The second electrode comprises a second major portion traversing the length of the substrate and a finger extending from the second major portion, wherein the first electrode finger extends toward the second electrode major portion and the second electrode finger extends toward the first electrode major portion and is substantially parallel to the first finger. A third electrode is connected to a ground, wherein the third electrode is interposed between and about the first and second electrodes. The process further includes applying an oscillating voltage source to the first electrode, generating an electrical field between the first electrode finger and the second electrode finger, wherein the electrical field extends into the fluid and monitoring a current passing to the ground from the second electrode.
The above described and other features are exemplified by the following figures and detailed description.